Work with the calcium-dependent cGMP phosphodiesterase is currently focused on mechanisms for regulation of calmodulin activity. Cyclic nucleotide phosphodiesterases in several tissues have been studied to characterize their regulatory properties. In other studies, we have found that with the differentiation of 3T3-L1 fibroblasts into cells having morphological and biochemical characteristics of adipocytes is the appearance of a particulate phosphodiesterase which exhibits a high affinity for cAMP and cGMP.